When a ceramic is used as a structural material, the structure must be a combination with another material, such as one based on a metal. With a combination structure such as this, a strain differential occurs as a result of the difference in the thermal and mechanical properties of the ceramic and the other material. In particular, this strain differential causes stress that bears on stationary parts under harsh binding conditions, and also causes the fracture that originates in this stress. Increasing the breaking strain of a material is effective at avoiding this type of ceramic fracture.
In the past, increased strength was essential to an increase in breaking strain. The following are a few examples of the disclosure of typical technology developed with this goal in mind.
Higher strength in a silicon nitride ceramic has been achieved by preventing defects from being introduced in the manufacturing process and by reducing the size of the particles. For example, as seen in "Journal of the Ceramic Society of Japan, 1034!, pp. 407-408 (1995)," there have been reports of the development of materials with high strength by adding a sintering auxiliary that inhibits grain growth and carefully performing the sintering at a temperature at which particles will not grow. Also, as seen in "Journal of the Ceramic Society of Japan, 97, pp. 872-874 (1989)," high-strength silicon nitride has been obtained by blending in as a sintering auxiliary a component that becomes a solid solution inside the silicon nitride particles during sintering, and simultaneously controlling sintering and dissolution of it.
The above methods have the following drawbacks, however, and a solution to these problems urgently needs to be found.
Extremely precise process control, as seen in the above example, is necessary in order to prevent defects from being introduced in the manufacturing process and to reduce the size of the particles. For instance, as seen in "Progress and Results of Next-Generation Research and Development of Fine Ceramics, Edited by the Fine Ceramics Research Association," increasing strength requires a thorough investigation into many factors, as well as repeated and tremendous quantities of experimentation and analysis to find solutions one after another. Because of this, greater breaking strain through increased strength poses problems in terms of higher cost and inferior reproducibility, which is a major obstacle to industrial utilization.
In the midst of this situation, the inventors conducted research with the above-mentioned prior art in mind and with the aim being the development of a silicon nitride whose strength would be maintained and would not depend on the manufacturing process, whose modulus of elasticity would be decreased, and whose breaking strain would be increased. In particular, increasing the breaking strain by lowering the modulus of elasticity is an important point not found in the above examples, and this offered the possibility of an easy solution to the problems that could not be solved with prior art.
There are two methods for lowering the modulus of elasticity of a ceramic. One method is to control the elastic behavior inherent in a ceramic substance, such as the formation of a solid solution. Since this method involves the control of the inherent characteristics of a substance, it is a universal control method, but there is a limit to how much the modulus of elasticity can be lowered since the solid solution system and solid solution amount are limited with a ceramic, and particularly the silicon nitride that is the object material of the present invention, which has strong covalent bonds. The other method is to compound a low modulus phase. The goal with a conventional compound material was to compound a strengthening phase whose modulus of elasticity and strength were both higher than those of the matrix, and to increase strength and toughness. However, when a low modulus phase is compounded, since the strength of a low modulus phase is generally low, the application of a common compounding rule to strength results in a decrease in strength along with the modulus of elasticity of the compound material.
In view of this, the inventors attempted the production of a high-strength porous silicon nitride in which the backbone consisted solely of oriented rodlike grains of silicon nitride, which are effective at increasing the strength of silicon nitride, and the other portion consisted solely of pores, which are effective at lowering the modulus of elasticity. The orientation of the rodlike grains is accompanied by the pores in the porous structure of the present invention being oriented in the same way as the grains, so there are fewer defects that would hinder strength manifestation than with ordinary spherical pores or the like. As a result, the present invention was perfected upon discovering that higher strength through the orientation of the rodlike grains can be realized simultaneously with a lower modulus of elasticity through the introduction of pores, and that breaking strain can be greatly increased.
Furthermore, as a result of in-depth investigation, it was discovered that the rodlike grains in the porous silicon nitride must have a minor axis diameter of 0.5 to 10 .mu.m and an aspect ratio of 10 to 100, and the porosity must be controlled to between 5 and 30% for a low modulus of elasticity and high strength to be manifested.
The porous material pertaining to the present invention has a structure in which the pores, which tend to be the fracture starting point, which is the most important aspect in the fracture of a ceramic, are supported by oriented rodlike grains. It was found that, as a result, the common compounding rule applies to the modulus of elasticity, but does not apply to strength, and strength can generally be maintained despite the introduction of pores that have an adverse effect on strength.